The meaning is clear, but why did the author use the distributive numbers quaternis and vicenis? I would have expected the ordinary cardinals quattuor and viginti. According to Allen & Greenough's New Latin Grammar (§ 136, 137) the distributive answers to the questions how many of each? or how many at a time?. If we were talking about a yearly decrease of effluents, the use of distributives would be understandable, but here? Does not the distributive imply two or more instances, which is not the case in this example because the period of time between the 1990's and 2007 as a whole is being considered.

Some more examples from the Nuntii Latini Finnici:

Catenae mercatoriae Americanae rettulerunt venditiones denis vel vicenis centesimis (10-20%) cecidisse: The use of distributives in this example seems to imply (at least to me) that sales fell by 10-20% for each of the retail chains. However, I believe that what is actually meant is that the sales for retail chains considered as a whole fell by 10-20%. But why then the distributive numbers?

Huc accedit, quod emissiones carbonei dioxydati iam sexennio proxime futuro vicenis centesimis (20%) deminuentur: Here vicenis seems to mean that there is an annual decrease by 20%, whereas it is more likely planned that the emissions are planned to fall by 20% in the 6-year period as a whole.

a percentage = centesima pars vel centesima [pars]2 percent = centesimae binae or two times decimal parts or two decimal parts at a time / togetherTo have fallen by how many percentage parts [//by how many times a percentage part at a time]? to have fallen by four percentage parts at a time [//by four times a percentage part].Quotiens centesimis [partibus] cecidisse? Quaternis centesimis [partibus] cecidisse.

adrianus wrote:a percentage = centesima pars vel centesima [pars]2 percent = centesimae binae or two times decimal parts or two decimal parts at a time / togetherTo have fallen by how many percentage parts [//by how many times a percentage part at a time]? to have fallen by four percentage parts at a time [//by four times a percentage part].Quotiens centesimis [partibus] cecidisse? Quaternis centesimis [partibus] cecidisse.

Yes, something like that is probably the solution to this conundrum. All other examples use the distributives as subjects or objects. But that does not make it easier for me to wrap my head around it, because it seems to be against the rules as explained either by A&G (New Latin Grammar) or Charles E. Bennett (also New Latin Grammar--not very inventive folk those grammarians ,-).